The objectives of the proposed work are to understand the assembly of the bacteriophage T4 head, particularly the mechanism of DNA packaging into the head, and control over initiation of prehead assembly; and the initiation of infection of the host cell by bacteriophage T4. A summary of recent major approaches to this problem is as follows: Genetic work has been extended on a cold sensitive mutation which blocks DNA packaging and which is located in the structural gene for a minor capsid protein-p20-situated at the neck of the phage head. Order of function studies show DNA packaging follows protein cleavage, and is not coupled to T4 DNA synthesis, nor probably to head expansion, but is dependent upon sealing single-stranded breaks in the DNA. In addition, the same capsid gene product-p20-plays a distinct, early role in initiating assembly of the prehead. This role has been defined by isolating new mutations in T4 gene 40 and gene 20 which indicated that p20 and p40 interact, and that p40 mediates p20 assembly upon the host membrane. In addition, a new method of investigating the dynamics of DNA packaging has been devised utilizing the phage T4 glucosylation system to distinguish packaged from cytoplasmic DNA in head intermediates. These experiments have lead to a model for DNA packaging: that DNA is actively packaged by an enzymatic mechanism through proteins situated at the apex of the head which pump the DNA into the head by inducing torsion in the DNA molecule.